We are attempting to gain insight into the biochemical processes of cell migration along a chemical gradient. While cells such as leucocytes, bacteria, and the cellular slime molds all show chemotactic behavior, we have focused upon the leucocyte. Efforts have proceeded in two directions: the characterization of leucoattractants released by bacteria and studies on the initial interaction between the attractant and the cell. Bacterial factors are heterogeneous, low molecular weight peptides, and it has been found that small formylmethionyl peptides, possibly related to the bacterial factors, are potent attractants for leucocytes. It is proposed to characterize further the bacterial factors and to investigate the mode of action of the well characterized fMet peptides on cellular migration. Obtaining such information may help to define further inflammatory events in general.